General Composition of the Blood.Blood consists of a faintly yellow fluid, the plasma or liquor sanguinis, in which are suspended numerous minute particles, the blood corpuscles, the majority of which are colored and give to the blood its red tint. If a drop of blood be placed in a thin layer on a glass slide and examined under the microscope, a number of these corpuscles will be seen floating in the plasma.

The Blood Corpuscles are of three kinds: (1) colored corpuscles or erythrocytes; (2) colorless corpuscles or leucocytes; (3) blood platelets.

1. Colored or red corpuscles (erythrocytes), when examined under the microscope, are seen to be circular disks, biconcave in profile. The disk has no nucleus, but, in consequence of its biconcave shape, presents, according to the alterations of focus under an ordinary high power, a central part, sometimes bright, sometimes dark, which has the appearance of a nucleus (Fig. 453,a). It is to the aggregation of the red corpuscles that the blood owes its red hue, although when examined by transmitted light their color appears to be only a faint reddish yellow. The corpuscles vary slightly in size even in the same drop of blood, but the average diameter is about 7.5μ,1 and the thickness about 2μ. Besides these there are found certain smaller corpuscles of about one-half of the size just indicated; these are termed microcytes, and are very scarce in normal blood; in diseased conditions (e. g., anemia), however, they are more numerous. The number of red corpuscles in the blood is enormous; between 4,000,000 and 5,000,000 are contained in a cubic millimetre. Power states that the red corpuscles of an adult would present an aggregate surface of about 3000 square yards.

If the web of a living frogs foot be spread out and examined under the microscope the blood is seen to flow in a continuous stream through the vessels, and the corpuscles show no tendency to adhere to each other or to the wall of the vessel. Doubtless the same is the case in the human body; but when human blood is drawn and examined on a slide without reagents the corpuscles tend to collect into heaps like rouleaux of coins (Fig. 453,b). It has been suggested that this phenomenon may be explained by alteration in surface tension. During life the red corpuscles may be seen to change their shape under pressure so as to adapt themselves, to some extent, to the size of the vessel. They are, however, highly elastic, and speedily recover their shape when the pressure is removed. They are readily influenced by the medium in which they are placed. In water they swell up, lose their shape, and become globular (endosmosis) (Fig. 453,c). Subsequently the hemoglobin is dissolved out, and the envelope can barely be distinguished as a faint circular outline. Solutions of salt or sugar, denser than the plasma, give them a stellate or crenated appearance (exosmosis) (Fig. 453,d), but the usual shape may be restored by diluting the solution to the same tonicity as the plasma. The crenated outline may be produced as the first effect of the passage of an electric shock: subsequently, if sufficiently strong, the shock ruptures the envelope. A solution of salt, isotonic with the plasma, merely separates the blood corpuscles mechanically, without changing their shape. Two views are held with regard to

Note 1. A micromillimetre (μ) is 1/1000 of a millimetre or 1/25000 of an inch. [back]